In recent years, there is a significant technological advance in miniaturization and weight savings of communication devices such as mobile phones and the like. As a filter used in such a communication device, there is used an elastic wave device which can be miniaturized, such as a surface acoustic wave device or an elastic boundary wave device. Moreover, an elastic wave device having an unbalanced-to-balanced function, i.e., a so-called balun function, is used in many cases as an elastic wave device for an RF (radio frequency) circuit of a mobile phone. Such an elastic wave device having an unbalanced-to-balanced function preferably has a good balance.
Herein, the balance is an index indicating the degree of idealness of an electric signal output from a balanced terminal of the elastic wave device having the unbalanced-to-balanced function in terms of opposite phase and same amplitude. The balance is expressed by two amounts, an amplitude balance and a phase balance. The amplitude balance is an amplitude ratio between two balanced signals, and its ideal value is 0 [dB]. The phase balance is a value obtained by subtracting 180° from a phase difference between two balanced signals, and its ideal value is 0°. As another evaluation method for balances, there is a common mode characteristic. If an ideal balance is achieved, signals between balanced terminals are completely cancelled. Therefore, the balance is good when an output signal is as small as possible (greatly attenuated) according to the transmission characteristics of the common mode characteristic. Thus, the common mode characteristic is an index for comprehensively evaluating the balance using the amplitude balance and the phase balance.
As the elastic wave device having the unbalanced-to-balanced function, a three-electrode longitudinally coupled elastic wave filter having three comb electrode pairs is used in many cases. With the three-electrode longitudinally coupled elastic wave filter, a standing wave with asymmetrical displacement distribution of an excited surface wave is formed. Accordingly, a current flowing through a comb electrode pair also becomes asymmetrical, and there is a problem regarding the balance. Efforts are being made to overcome the problem of such a three-electrode longitudinally coupled elastic wave filter. For example, a region where an electrode finger pitch is small is provided at a portion of adjacent comb electrode pairs in the three-electrode longitudinally coupled elastic wave filter. Further, the electrode finger pitch is made different for left and right for the region where the electrode finger pitch is made small. In this manner, a technique regarding asymmetrical design and a technique regarding weighting are being proposed.
Furthermore, it is preferable that an insertion loss of a filter used for an RF unit of a mobile phone is made as small as possible. As a technique of reducing the insertion loss, there is proposed a technique of reducing the loss by comb electrode resistance by connecting the three-electrode longitudinally coupled elastic wave filters in parallel, for example.
As cited references related to the invention of this application, Patent Literature 1 and Patent Literature 2 are known, for example.